Compressible fixation apparatus for spinal surgery

ABSTRACT

A spinal fixation apparatus is configured with a pair of endpieces and an intermediate compressible portion. The compressible portion may comprise a pair of mating sliders comprising toothed arms attached to the endpieces. A gear engages the toothed arms. When the gear is rotated, the sliders bring the endpieces together, compressing the apparatus.

PRIORITY INFORMATION

This application is a continuation of related U.S. Application Ser. No.10/628,079, filed Jul. 24, 2003, which claims priority to U.S.Provisional Application No. 60/398,623, filed on Jul. 24, 2002. Thedisclosures of the above-identified applications are hereby incorporatedby reference in their entireties.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of medicalequipment, specifically to a device for use in spinal surgery such as adiskectomy and spinal fusion.

2. Description of the Related Art

Anterior cervical diskectomy and corpectomy have been utilized for thetreatment of cervical radiculopathy, myelopathy, and myeloradiculopathyfor over 50 years. The traditional methods for treatment of the singlelevel and two-level decompressions have been the Cloward technique andthe Smith-Robinson technique. Both techniques typically involve removingthe disk and a portion of the end plate and inserting a graft. In theCloward technique, a circular bone dowel of autologous or allograft boneis inserted into the defect in order to fuse the two adjacent vertebrae.In the Smith-Robinson technique, a horseshoe shaped graft is takeneither from the patient's iliac crest or from allograft bone andinserted in the interspace.

The literature over the years shows that such fusions have a relativelypoor success rate without stabilization with plate fixation. Some of thereasons for failure include pesudarthrosis, poor fixation (improperplacement of the screws), improper placement of the graft, poor fixationof the graft, poor preparation of the cartilaginous end plates, andimproper placement of the plate. As fusions, and especially multi-levelfissions become more common, the need to improve the fusion rate inthese surgeries is becoming more pronounced.

SUMMARY OF THE INVENTION

In one embodiment, the invention comprises an apparatus for theenhancement of fusion of at least two adjacent vertebrae comprising atleast a superior and an inferior endpiece, wherein the superior endpieceis adapted to be affixed to a superior vertebral body, and the inferiorendpiece is adapted to be affixed to an inferior vertebral body. Theapparatus farther comprises a compressible portion located between thesuperior and inferior endpieces. The compressible portion may comprise asliding mechanism.

In another embodiment, the invention comprises a method of enhancingfusion between vertebral bodies comprising accessing a spinal portion,affixing respective endpieces to superior and inferior vertebral bodies,adjusting the distance between the endpieces into a compressed position,and locking the apparatus in the compressed position.

In another embodiment, the invention comprises a method of manufacturingan apparatus for the enhancement of fusion between at least two adjacentvertebrae comprising attaching a compressible coupling to a superior andan inferior endpiece, wherein the superior and inferior endpieces areadapted to be affixed to a superior and an inferior vertebral body,respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a compressible spinal fixation deviceaccording to one embodiment of the invention.

FIG. 2 is an illustration of a specific embodiment of a compressibleportion of a spinal fixation device.

FIG. 3 is an illustration of another embodiment of a compressiblefixation device.

FIG. 4 is a cross sectional view along line A-A of the device of FIG. 3.

FIG. 5A is a perspective illustration of one embodiment of acompressible spinal fixation device.

FIG. 5B is a cross-sectional view of the compressible device of FIG. 1A,taken along line A-A of FIG. 5A.

FIG. 5C is a cross-sectional view of the compressible device of FIG. 1A,taken along line B-B of FIG. 5A.

FIGS. 6A-D illustrate one embodiment of the endpiece of the device ofFIGS. 5A-C.

FIGS. 7A-D illustrate one embodiment of the housing of the device ofFIGS. 5A-C.

FIGS. 8A-G illustrate one embodiment of a slider of the device of FIGS.5A-C.

FIG. 9 is a perspective illustration of an additional embodiment of acompressible spinal fixation device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the invention will now be described with reference to theaccompanying Figures, wherein like numerals refer to like elementsthroughout. The terminology used in the description presented herein isnot intended to be interpreted in any limited or restrictive manner,simply because it is being utilized in conjunction with a detaileddescription of certain specific embodiments of the invention.Furthermore, embodiments of the invention may include several novelfeatures, no single one of which is solely responsible for its desirableattributes or which is essential to practicing the inventions hereindescribed.

The fixation plate described herein differs from existing plates byallowing compression of the graft, and fixation of the plate in thevertebral bodies. Compression of the graft will improve fixation andreduce failure rates. According to Wolff's law, bone that is undercompression will fuse. Currently, plates are placed without compression.This practice may create a permanent distraction component which couldultimately result in failure for osteointegration. On the other hand,the compressible plates described herein allow compliance with Wolff'slaw, thereby significantly improving the chances of successful fusion.Thus, the compressible fixation plate described herein offers importantadvantages over present-day plate fixation. These plates are especiallyuseful for cervical spinal surgery, but can be used on all portions ofthe spine with appropriate sizing.

One embodiment of a compressible spinal fixation plate is illustrated inFIG. 1. As illustrated in this Figure, a spinal fixation apparatus inaccordance with this embodiment includes superior and inferior endpieces10, 12 which advantageously include some form of fixation structuressuch as screw holes for attachment to respective upper and lowervertebral bodies. Located intermediate to the endpieces is acompressible portion 20. In operation, the endpieces may be affixed torespective upper and lower vertebral bodies, with one or more diskspaces positioned in between. After installation, the compressibleportion 20 is compressed such that the two endpieces are forced towardone another, compressing the intermediate disk spaces in the process.This in turn compresses any graft that has been placed within the diskspace(s), and improves the fusion process.

It will be appreciated that a variety of implementations of acompressible region 20 may be implemented. FIG. 2 illustrates a spinalfixation device that incorporates a specific embodiment of thecompressible region 20 of FIG. 1. In this embodiment, the upper endpiece30 includes a pair of arms 32, 34, to form a generally U-shaped body.The lower endpiece 31 also includes a pair of arms 36, 38 to form agenerally U-shaped body. The respective arm pairs face each other andmate in a slidable fashion forming a structure with a pair of outer arms34, 36 which surround and slide along a pair of inner arms 32, 38.

The inwardly directed faces of the inner arms 32, 38 include teeth 40,42 respectively. These teeth mate with the teeth of a central gear 44.Thus, when the gear 44 is rotated counter-clockwise, the endpieces 30,31 are drawn toward each other in the direction of arrows 46, 48. Thisreduces the overall length of the device by compressing this middleregion of the apparatus, and accordingly compresses any intermediatedisk space as described above.

FIG. 3 illustrates an embodiment similar to that shown in FIG. 2. Inthis embodiment, the gear 44 is captured within a two piece housing 47,49 that captures the arms and gear in the central region of the device.The housings can be held together with screws from the rear, forexample. In some embodiments, a hole for a set screw (not shown, but seealso FIG. 5) is placed through the housing pieces 47, 49, such that whenthe set screw is installed, the gear is prevented from rotating, therebylocking the device in the compressed position after the gear is rotatedto set the overall compressed device length after installation.

FIG. 3 also illustrates an alternative configuration for the attachmentof the arm segments 32, 34, 36, 38 to the endpieces 30, 31. In theembodiment of FIG. 2, the endpieces are integral with the arm segments.In the embodiment of FIG. 3, however, the endpieces 30, 31 contain anopening 41 which engages a hooked flange 43 provided as part of aseparate slider piece that incorporates the arm segments. With thisdesign, the endpieces can be attached to their respective vertebralbodies first, and the central commpressible region can be attached tothem after endpiece installation. It will be appreciated that otherboding mechanisms between endpieces and slider pieces may be providedsuch as screws, rivets, etc.

For multi-level fusions, the device in FIG. 3 can be modified such thatthe lower housing portion 47 includes flanges and ports for bone screwssuch that the housing portion 47 can be affixed to an intermediatevertebral body. Multi-level fusions can also be accommodated by hookinga second slider piece into one of the openings 41 in one of the endpieces. This would then engage with another slider above or below suchthat three endpieces (an upper, lower and intermediate) are joined by apair of adjacent compressible segments. In this case, the intermediateendpiece would have two sliders engaged in its central opening 41.Alternatively, separate fixing plates may be attached to the slider armsthemselves.

FIG. 4 is a cross section along line A-A of the device shown in FIG. 3.In this embodiment, the sides of the arm segments are angled or beveledto provide a mating surface that resists relative vertical displacementof the arms with respect to one another. In some advantageousembodiments, the bevel angle (θ) of the arm segments where they contactthe housing 47 is different from the bevel angle (100 ) of the armsegments where they contact each other. This facilitates capture of thearms within the housing and to each other. In one specific embodimentthe angle θ is about 30 degrees and the angle φ is about 25 degrees.

A more detailed illustration of one specific embodiment of acompressible plate 50 is illustrated in FIG. 5A. In this embodiment, theapparatus 50 comprises two endpieces 52A-B, wherein each endpiece 52A-Bis configured for attachment to a vertebral body with bone screws, forexample, via a plurality of apertures 54. Ports 58A-B in the endpieces52A-B link the endpieces 52A-B to a sliding compression mechanism 56,wherein the sliding mechanism 56 is designed to compress the vertebralbodies to which the endpieces 52A-B are attached.

The sliding compression mechanism 56 comprises a first slider 62 and asecond slider 64 each having an approximately U-shaped geometry, whereinat least one arm of each slider 62, 64 has a plurality of teeth 66 forengaging a gear (not shown). In the embodiment shown, each slider 62, 64has a flange 68 configured to engage the port 58A-B on the endpiece52A-B. In an alternative embodiment, the sliders may be integral to theendpieces. The arms of the slider pieces 62,64 are configured to slideover each other within the housing 70. Shown are first and secondsliders 62, 64 designed so that each slider has a toothed arm and anon-toothed arm, wherein the toothed and non-toothed arms are parallelto each other. In the embodiment shown, the first and second sliders 62,64, are configured such that the toothed arm of first slider 62 slidesover the non-toothed arm of second slider 64, and the toothed arm of thesecond slider 64 slides over the non-toothed arm of first slider piece62.

In the embodiment shown in FIG. 5A, housing 70 comprises a gear hole 72for operation of the sliding mechanism 56. Before or after installingthe device, a gear is inserted into hole 72 so as to engage the teeth 66of each slider piece. The gear is rotated with a tool, causing theslider arms to move relative to one another, thereby pulling theendpieces 52A-B towards each other, and compressing the bone graft. Ahole 74 for a set screw is also provided in this embodiment. Whentightened, the set screw clamps the housing over the mated arms of thesliders 62, 64, securing the device in the compressed position. In thisembodiment, the gear can be removed and the apparatus will remain fixedin the compressed position. In either this or the embodiments previouslydescribed, a ratcheting mechanism may also be installed in conjunctionwith the gear. In this embodiment, as gear turning engages the ratchet,the device will stay in the compressed position such that prior toinstalling the set screw, or if the surgeon wants to loosen the setscrew, the device won't expand back to an elongated position.

FIG. 5B is a cross-sectional view of cervical plate 50 taken along lineA-A of FIG. 5A. In this embodiment the sliders 62, 64 each has flange 68at one end of the parallel arms. Flanges 68 extend through ports 58A-Bof endpieces 54A-B. The arms of sliders 62, 64 are shown with lips thatextend around the bottom face of apertures 58A-B. This designfacilitates attachment of the whole compression mechanism 56 toendpieces 52A-B. Housing 70 is shown with gear hole 72 providing anopening through housing 70 to expose the teeth 66 of the slider piece,making teeth 66 available for engagement with a gear (not shown).

FIG. 5C is a cross-sectional view of cervical plate 50 taken along lineB-B of FIG. 5A. Toothed arm of first slider 62 is shown extended overnon-toothed arm of second slider 64. The teeth 66 of first slider areexposed in gear hole 72. Parallel to the toothed arm of first sliderpiece 62 is shown the toothed arm of second slider piece 64. The toothedand non-toothed arms fit together in approximately the same shape of twoL's wherein the long ends of each L rest against each other, andhorizontally within housing 70. The shorter ends of each L are parallelto each other and are on opposite sides of the long ends of each L. Theshorter ends comprise the outer edge of the non-toothed arm, and theinner, toothed-edge of the toothed arm of the mating arms. The inneredge of each slider comprises teeth 66. Teeth 66 face inwards towardsthe other arm of the same slider piece, and are exposed in gear hole 72.The outer edge of each slider runs along the inside wall of housing 70.

FIGS. 6A-6D, illustrate in more detail the endpieces 52A-B. FIG. 6A is aperspective view of the end plate 52, showing apertures 54, and port 58.As shown in FIG. 6B, endpieces 52A-B may be contoured around apertures54. FIG. 6C is a cross section of FIG. 6B taken along line A-A. In thisembodiment the top face round apertures 54 slope down towards the centerof the circle, providing a convenient mechanism to house the head of abone screw (not shown). FIG. 6D is a cross section of FIG. 6B takenalong line B-B. This drawing further illustrates the angle of the topface of apertures 54 designed to accommodate the head of the fasteningdevice.

FIGS. 7A-7D illustrate in more detail the housing 70 of the embodimentshown in FIG. 5A. 7A is a perspective view of housing 70. Hollows extendalong parallel to each other through housing 70. Each hollow isconfigured to accommodate a pair of mating arms of first and secondsliders 62, 64. Separating the two hollows is lock mechanism, such as aset crew mounting hole, 74 and gear hole 72. Apart from the hollows toaccommodate the arms of sliders 62, 64, the aperture 74 configured tohouse a lock, and gear hole 72, the housing may be one integral solidpart. The top face of housing 70 slopes inwards around the opening oflock mechanism 74, providing a convenient mechanism to house a lockingdevice, such as a screw (not shown).

FIG. 7B is a top plan view of housing 70. As seen in this figure, theopening in the top face of housing 70 comprising the top face gear hole72 is larger than the opening in the bottom face of housing 70comprising the bottom face of gear hole 72. FIG. 7C is a cross-sectionalview of FIG. 7B, taken along line A-A, along the diameter of gear hole74. FIG. 7C shows the openings in the top face and bottom face ofhousing 70 comprising gear hole 72. FIG. 7D is a cross-sectional view ofhousing 70 taken along line B-B.

FIGS. 8A-8D illustrate in more detail one of the slider pieces 62, 64.FIG. 8A is a perspective view of first slider piece 62. Shown is theapproximate U-shape of slider piece, 62, comprising parallel toothed andnon-toothed arms. Both arms are perpendicular to a back that comprisesflange 68 to facilitate attachment to the port 58A of endpiece 54. Whenfirst and second sliders 62, 64 are assembled together to form slidingcompression mechanism 56, each set of mating arms fits within a hollowof housing 70.

FIG. 8B is a top plan view of a sliders 62, 64. In this embodiment, thetoothed arms of sliders 62, 64 are configured to accommodate thenon-toothed arm of the mating slider underneath the toothed arm. Thenon-toothed arms of sliders.

FIG. 8C is a cross-sectional view of FIG. 8B along line A-A. This viewdepicts the arrangement of a toothed arm of first slider 62 mated with anon-toothed arm of second slider piece 64. Shown is the arrangement ofthe arms of sliders 62, 64, described above, wherein the arms fittogether in approximately the shape of two L's fit together. In thisconfiguration, the outer edge of the toothed arm does not contact thehousing. Rather, the edge opposite the teeth of the toothed arm runsalong an edge of the non-toothed arm of second slider piece 64.

FIGS. 8E-8G illustrate in more detail an embodiment of first sliderpiece 62. FIG. 8E is the same view of first slider piece 62 as FIG. 8B.FIG. 8F is a cross-sectional view of FIG. 8E taken along line C-C. Inthe embodiment shown, the junction of the arms and the backs of eachslider 62, 64 is angled. FIG. 8G is a cross-sectional view of FIG. 8Etaken along line D-D. As depicted in FIG. 8G, the junction of the armsand the back of the slider is angled.

An additional embodiment of a compressible plate is illustrated in FIG.9, wherein a plurality of lock housings are provided which attach to thearms of the sliders 62, 64 intermediate the endpieces 52A, 52B. Thisdevice may also be advantageous in a multi-level application where alonger device is needed.

The traditional approach to the anterior cervical spine may be the samefor the placement of the plate described herein when used in a cervicalapplication. A transverse or oblique incision is made on the right orleft side of the neck. The medial border of the sternocleidomastoid isretracted along with the carotid sheath. The trachea and the esophagusare retracted to the opposite side, providing surgical access down tothe prevertebral fascia. Sweeping in a cephalad and caudal directionenables access to the anterior vertebral column. This allowsidentification of vertebral bodies and disk spaces, including the diskor vertebral body affected by the pathology. Use of interoperativex-rays at this stage confirms the location of the pathology. A surgeonthen removes the affected disk(s) or vertebral body. If a diskectomy isperformed, following removal of the disk, some of the cartilaginous endplate may be denuded to allow better incorporation of bone on bonesurfaces in compression.

Those skilled in the art of orthopedic and neurosurgery can install thedevice described herein using standard surgical tools. The plate may,for example, be applied in a similar manner in the midline of theanterior cervical region spanning a distance of the graft. The presentinvention allows the surgeon to advantageously place the upper andcervical portions in the mid-substance of the vertebral bodies. Byaffixing the plate to the mid-substance of the vertebral bodies, thebest purchase in the bone as well as avoidance of abutment between theplates and adjacent disks is achieved. This placement minimizes thepossibility of adjacent segment disease or break out of the screws fromthe vertebral body and subsequent loosening of the graft. In short,placement of the screws in the upper and lower cervical vertebral bodiessuch that the graft is now capable of being compressed, provides thebest possible construct for ensuring a cervical fusion. The capabilityof compressing the graft as well as anchoring in the best bone superiorand inferior to the graft minimizes the risk for pseudarthrosis, graftmigration, and/or screw dislodgment.

The foregoing description details certain embodiments of the invention.It will be appreciated, however, that no matter how detailed theforegoing appears in text, the invention can be practiced in many ways.As is also stated above, it should be noted that the use of particularterminology when describing certain features or aspects of the inventionshould not be taken to imply that the terminology is being re-definedherein to be restricted to including any specific characteristics of thefeatures or aspects of the invention with which that terminology isassociated. The scope of the invention should therefore be construed inaccordance with the appended claims and any equivalents thereof.

1. A method of enhancing fusion of a graft in the cervical spinecomprising: accessing an anterior portion of cervical vertebrae;affixing endpieces of a compressible fixation device to the vertebralbodies on either side of a graft; adjusting the distance between saidendpieces into a compressed position, thereby compressing the graft; andlocking the device into the compressed position.
 2. The method of claim1, wherein the endpieces are affixed to the mid-substance of thevertebral bodies.
 3. The method of claim 1, wherein said affixingcomprises screwing the endpieces to the vertebral bodies.
 4. The methodof claim 1, wherein said adjusting comprises rotating a gear that mateswith teeth located on the endpieces to draw the endpieces toward eachother.
 5. The method of claim 4, where said locking is achieved byinstalling a set screw to prevent the gear from rotating.
 6. The methodof claim 1, wherein said adjusting comprises sliding the endpiecestoward each other.
 7. A method of enhancing fusion of a graft in amultilevel cervical vertebral fusion comprising: accessing an anteriorcervical spinal portion; removing some or all of at least two cervicalspinal disks to create a space between cervical vertebrae; placing atleast one graft in the space created by the removal of some or all ofthe at least two cervical spinal disks; affixing respective endpieces ofa compressible fixation device to vertebral bodies superior and inferiorto the graft; affixing at least one intermediate endpiece of acompressible fixation device to an intermediate vertebral body betweensaid superior and inferior vertebral bodies; adjusting the distancebetween said endpieces into a compressed position, thereby compressingthe graft; and locking the device into the compressed position.
 8. Themethod of claim 7, wherein the endpieces are affixed to themid-substance of the vertebral bodies.
 9. The method of claim 7, whereinsaid affixing comprises screwing the endpieces to the vertebral bodies.10. The method of claim 7, wherein said adjusting comprises rotating agear that mates with teeth located on the endpieces to draw theendpieces toward each other.
 11. The method of claim 10, where saidlocking is achieved by installing a set screw to prevent the gear fromrotating.
 12. The method of claim 7, wherein said adjusting comprisessliding the endpieces toward each other.